1. Field of the Invention
The invention is directed to a variable flow control valve having a check valve attached thereto and a method for using the same.
2. Description of the Related Art
During the operation of a municipal reverse osmosis water purification plant, a concentrate control valve (CCV) is utilized to control the pressure and flow rate of unpurified water through a reverse osmosis membrane. This reverse osmosis system is generally built using large blocks of membrane filters called trains. Multiple trains are connected in parallel to achieve the required capacity. Each train may have a capacity ranging from less than a hundred thousand gallons per day to more than one million gallons per day. The trend has been to increase the capacity of each individual train rather than to build higher volumes of trains to meet increasing capacity requirements. Each train requires one concentrate control valve. These concentrate control valves are required to operate in three modes, including                1. An operating mode in which the valve should create a pressure drop capable of being modulated at varying flow rates.        2. Start up and flushing modes wherein the valve should create a minimum pressure drop.        3. A shut down mode in which the valve must shut drip tight.        
Currently there are three concentrate control valve designs being utilized. A globe style control valve which is intentionally oversized to meet the minimum pressure drop requirements and during normal operation, the globe valve operates toward the lower end of its travel to create the required pressure drop, but this produces cavitation. Cavitation in a valve creates excessive wear and noise. A special anti-cavitation globe is available, however the price is excessive. As a second option a v-port control ball valve may be used and is preferred over the globe style control valve. Additionally, an actuated butterfly valve may be substituted for the v-port ball valve and either valve may be utilized in conjunction with an orifice plate to limit the pressure drop across the valve to eliminate cavitation. However, during start up and flushing, when using a v-port control valve or a butterfly valve, the minimum pressure drop is increased due to the presence of the orifice plate.
A design is required for a control valve that is relatively inexpensive and provides for variable flow and variable pressure drops past the valve without cavitation but at the same time incorporates a check valve to close the valve and prevent backflow.